Bottom Line:
Quercetin was less effective, with a 50% decline.Quercetin 3- and 7-O-sulphate had no effect on PGE-2.The results indicate that quercetin may exert its pharmacological effects, at least in part, via its metabolites.

ABSTRACTQuercetin (3,5,7,3',4'-pentahydroxyflavone) is a flavone with putative ability to prevent cancer and cardiovascular diseases. Its metabolism was evaluated in rats and human. Rats received quercetin via the intravenous (i.v.) route and metabolites were isolated from the plasma, urine and bile. Analysis was by high-performance liquid chromatography and confirmation of species identity was achieved by mass spectrometry. Quercetin and isorhamnetin, the 3'-O-methyl analogue, were found in both the plasma and urine. In addition, several polar peaks were characterised as sulphated and glucuronidated conjugates of quercetin and isorhamnetin. Extension of the metabolism studies to a cancer patient who had received quercetin as an i.v. bolus showed that (Quercetin removed) isorhamnetin and quercetin 3'-O-sulphate were major plasma metabolites. As a catechol, quercetin can potentially be converted to a quinone and subsequently conjugated with glutathione (GSH). Oxidation of quercetin with mushroom tyrosinase in the presence of GSH furnished GSH conjugates of quercetin, two mono- and one bis-substituted conjugates. However, these species were not found in biomatrices in rats treated with quercetin. As cyclo-oxygenase-2 (COX-2) expression is mechanistically linked to carcinogenesis, we examined whether quercetin and its metabolites can inhibit COX-2 in a human colorectal cancer cell line (HCA-7). Isorhamnetin and its 4'-isomer tamarixetin were potent inhibitors, reflected in a 90% decrease in prostaglandin E-2 (PGE-2) levels, a marker of COX-2 activity. Quercetin was less effective, with a 50% decline. Quercetin 3- and 7-O-sulphate had no effect on PGE-2. The results indicate that quercetin may exert its pharmacological effects, at least in part, via its metabolites.

Mentions:
In the light of the nephrotoxic manifestations of quercetin observed in a clinical trial (Ferry et al, 1996), the hypothesis was tested such that quercetin might undergo metabolic oxidation in species reacting with GSH conjugation to furnish moieties that could potentially be targeted specifically to the kidney. To this end, quercetin was incubated with mushroom tyrosinase, which can oxidise catechols to their quinones (Duckworth and Coleman, 1970). High-performance liquid chromatography analysis of an extract of the reaction mixture furnished seven peaks eluting prior to quercetin (Figure 5Figure 5

Mentions:
In the light of the nephrotoxic manifestations of quercetin observed in a clinical trial (Ferry et al, 1996), the hypothesis was tested such that quercetin might undergo metabolic oxidation in species reacting with GSH conjugation to furnish moieties that could potentially be targeted specifically to the kidney. To this end, quercetin was incubated with mushroom tyrosinase, which can oxidise catechols to their quinones (Duckworth and Coleman, 1970). High-performance liquid chromatography analysis of an extract of the reaction mixture furnished seven peaks eluting prior to quercetin (Figure 5Figure 5

Bottom Line:
Quercetin was less effective, with a 50% decline.Quercetin 3- and 7-O-sulphate had no effect on PGE-2.The results indicate that quercetin may exert its pharmacological effects, at least in part, via its metabolites.

ABSTRACTQuercetin (3,5,7,3',4'-pentahydroxyflavone) is a flavone with putative ability to prevent cancer and cardiovascular diseases. Its metabolism was evaluated in rats and human. Rats received quercetin via the intravenous (i.v.) route and metabolites were isolated from the plasma, urine and bile. Analysis was by high-performance liquid chromatography and confirmation of species identity was achieved by mass spectrometry. Quercetin and isorhamnetin, the 3'-O-methyl analogue, were found in both the plasma and urine. In addition, several polar peaks were characterised as sulphated and glucuronidated conjugates of quercetin and isorhamnetin. Extension of the metabolism studies to a cancer patient who had received quercetin as an i.v. bolus showed that (Quercetin removed) isorhamnetin and quercetin 3'-O-sulphate were major plasma metabolites. As a catechol, quercetin can potentially be converted to a quinone and subsequently conjugated with glutathione (GSH). Oxidation of quercetin with mushroom tyrosinase in the presence of GSH furnished GSH conjugates of quercetin, two mono- and one bis-substituted conjugates. However, these species were not found in biomatrices in rats treated with quercetin. As cyclo-oxygenase-2 (COX-2) expression is mechanistically linked to carcinogenesis, we examined whether quercetin and its metabolites can inhibit COX-2 in a human colorectal cancer cell line (HCA-7). Isorhamnetin and its 4'-isomer tamarixetin were potent inhibitors, reflected in a 90% decrease in prostaglandin E-2 (PGE-2) levels, a marker of COX-2 activity. Quercetin was less effective, with a 50% decline. Quercetin 3- and 7-O-sulphate had no effect on PGE-2. The results indicate that quercetin may exert its pharmacological effects, at least in part, via its metabolites.